Class and object changes

instanceof, is_a(),
is_subclass_of() and catch

In PHP 5.0, is_a() was deprecated and replaced by the
instanceof operator. There were some issues with the
initial implementation of instanceof, which relied on
__autoload() to search for missing classes.
If the class was not present, instanceof would throw
a fatal E_ERROR due to the failure of
__autoload() to discover that class. The same behaviour
occurred in the catch operator and the
is_subclass_of() function, for the same reason.

None of these functions or operators call __autoload()
in PHP 5.1.x, and the class_exists() workarounds used
in code written for PHP 5.0.x, while not problematic in any way, are no
longer necessary.

Abstract private methods

Abstract private methods were supported between PHP 5.0.0 and PHP 5.0.4,
but were then disallowed on the grounds that the behaviours of
private and abstract are mutually
exclusive.

Access modifiers in interfaces

Under PHP 5.0, function declarations in interfaces were treated in exactly
the same way as function declarations in classes. This has not been the
case since October 2004, at which point only the public
access modifier was allowed in interface function declarations. Since
April 2005 - which pre-dates the PHP 5.0b1 release - the
static modifier has also been allowed. However, the
protected and private modifiers will
now throw an E_ERROR, as will
abstract. Note that this change should not affect your
existing code, as none of these modifiers makes sense in the context of
interfaces anyway.

Changes in inheritance rules

Under PHP 5.0, it was possible to have a function declaration in a derived
class that did not match the declaration of the same function in the base
class, e.g.